Biofuels are of increasing interest for a number of reasons including: (1) they are a renewable resource, (2) their production is less dependent on geopolitical considerations, (3) they provide the possibility of a direct replacement of petroleum-based fuels in existing vehicles, and (4) the net greenhouse gas emissions can be substantially reduced by virtue of CO2 uptake by biofuel precursors—particularly in the case of cellulosic feedstocks. See Pearce, “Fuels Gold,” New Scientist, 23 Sep., pp. 36-41, 2006.
An easily-obtainable biofuel is vegetable oil, which largely comprises triglycerides and some free fatty acids. The properties of vegetable oil, however, make it generally inappropriate for use as a direct replacement for petroleum diesel in vehicle engines, as the vegetable oils' viscosities are generally too high and do not burn cleanly enough, thereby leaving damaging carbon deposits on the engine. Additionally, vegetable oils tend to gel at lower temperatures, thereby hindering their use in colder climates. These problems are mitigated when the vegetable oils are blended with petroleum fuels, but still remain an impediment for long-term use in diesel engines. See Pearce, 2006; Huber et al., “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering,” Chem. Rev., vol. 106, pp. 4044-4098, 2006.
Transesterification is currently a method used to convert vegetable oils into diesel-compatible fuels (i.e., biodiesel) that can be burned in conventional diesel engines. When methanol is used to transesterify vegetable oil, the resulting biodiesel is primarily composed of methyl esters that have long straight chain aliphatic groups attached to a carbonyl group (i.e., fatty acid methyl esters, or FAME). Such biodiesel invariably comprises ester species having regions of unsaturation, i.e., double bonds, although the amount of such unsaturated ester species can vary widely depending upon its biomass source. See, e.g., Meher et al., “Technical aspects of biodiesel production by transesterification—a review”, Renewable & Sustainable Energy Reviews, vol. 10, pp. 248-268, 2006. While such processing of vegetable oil enhances their ability to be used as fuels, the resulting ester-based compositions still have low-temperature properties that are generally inferior to those of conventional petroleum-based diesel.
Accordingly, compositions and/or methods for improving the low-temperature properties of biodiesel made by transesterification of vegetable oils would be quite useful, particularly wherein they impart the resulting fuel with greater seasonal and geographic utility.